The present invention is related to an electrical outlet strip and, more particularly, to a combination electrical outlet strip which comprises a transient voltage surge suppression (TVSS) protection device with electromagnetic interference noise blocking capabilities to and from an attached electrical appliance.
Many consumer electronic goods used today contain electronic circuitry which includes miniaturized components and electrical contacts which, due to their diminutive size and the materials from which they are formed, are very susceptible to stray electrical energy caused by voltage transients and electromagnetic interference (EMI). Voltage transients typically occur on the incoming electrical power lines and are represented as a surge or spike in the amplitude of the incoming voltage. In a transient situation, large voltage potentials (i.e., significantly higher than normal operating voltage) may be present over a short time period. These voltage transients can severely damage or destroy electrical components or contacts, thereby rendering the electrical equipment inoperative.
EMI is an unwanted electrically transmitted signal that can propagate through electrical equipment and components interfering with their proper operation by altering their normal operating parameters. Examples of EMI interference include noise on the phone line, erratic data bits on a digital transmission, interference lines across a television screen, in addition to numerous other malfunctions. EMI generally appears in two forms: radiated and conducted. Radiated interference (i.e., Radio Frequency Interference, RFI) is interference that travels through the air from a source to a receiving device and is often the by-product of electromagnetic fields around external wiring. Conducted interference is an undesirable electromagnetic signal traveling on a medium such as an AC electrical power line or xe2x80x98wirexe2x80x99 which may be conducted into sensitive electronic equipment and cause malfunctions or otherwise affect operation of the equipment. The conducted EMI may originate from a source directly attached to the wire or it may originate as radiated EMI (i.e., RFI) that is received by and propagated along the wire. In this regard, the wire may act as an antenna that is able to transmit and receive RFI. Further, conducted EMI appears in two forms, common-mode and differential-mode. Common mode EMI occurs when unwanted signal(s) consisting of multiple currents flow in the same direction along the conducting wire. Differential-mode EMI interference occurs when unwanted signal(s) consisting of multiple currents flow in opposite directions along the conducting wire. Finally, EMI can occur in either broadband or narrowband frequencies. Broadband interference is interference which has broad spectral energy distribution exhibited over a wide frequency range and is generally a result of sudden changes in voltage or current. Narrowband interference is interference which has its principal spectral energy confined to a specific frequency or frequencies. This type of interference is usually produced by an oscillator circuit which contains energy only at the frequency of oscillation and harmonics of that frequency. In normal usage, the magnitude of the interference will depend on the bandwidth of the receiver used to detect the signals.
An important source of both conducted and radiated EMI is electrical equipment coupled to AC electrical lines such as computers, switching power supplies, and electrical devices which utilize electrical motors, such as refrigerators, air conditioners and treadmills, etc. In addition, these electronic devices can also produce radiated EMI which may be seen in other nearby electrical devices. As noted above, once EMI from an electrical device is conducted into an electrical wiring circuit, the wiring may act as an antenna and xe2x80x9cbroadcastxe2x80x9d the conducted EMI as RFI throughout a structure.
There are generally two different issues present regarding EMI and electronic devices, the EMI given off by a particular device and the EMI received by the device. Over the last several years, consumer electronic devices have become more sophisticated with the use of microprocessors whose functioning requires clean electrical power free of noise and frequency components that can interfere with their operation. Accordingly, these devices have become more susceptible to EMI problems. Susceptibility is defined as the characteristic which causes equipment to malfunction or exhibit an undesirable response when it is subjected to electromagnetic interference voltages or fields.
It is, therefore, a primary objective of the present invention to provide an apparatus that reduces the introduction of conducted EMI into an AC power system.
It is a further objective of the present invention to provide means to reduce conducted EMI in digital data transfer sensitive frequencies.
The inventors of the present invention have recognized that many currently used consumer electronic appliances and, particularly, motor driven devices produce EMI in a series of impulse spikes which are generally conducted into the AC power system. Further, the inventors have discovered that these EMI impulse spikes are often of such a magnitude that some consumer digital devices cannot function properly even when there is an EMI filter between the digital device and its AC power supply (i.e., outlet). The inventors have recognized that digital data transfer devices are especially susceptible to conducted EMI interference which often results in poor or lost data transfer and/or disconnection. In particular, broadband data transmissions in the frequency range of 25 kHz to 12 MHz such as telephony digital subscriber line (DSL) systems are highly susceptible to EMI interference produced by motor driven appliances. The inventors have recognized that the impulse spikes created by motor driven devices when switched on or off and, in some cases, created continuously as they operate are disruptive to the operation of data transfer devices and that suppression of these EMI impulse spikes from motor driven appliances before they are conducted into an AC wiring circuit is more effective than filtering the AC power as it is received by the digital device.
In light of these recognitions and in line with the above objectives, the present invention embodies an electrical outlet strip apparatus for interconnection between an AC power source and an electrical appliance that provides transient voltage surge suppression for the appliance and substantially reduces EMI to and from the appliance. The apparatus comprises a first input line, a second input line and a ground line which are adapted to be connected to an existing AC source (e.g., AC outlet). The apparatus further comprises at least one electrical connector having a first, second and third conductors connected to the first input line, the second input line and the ground line, respectively. This connector is operative to interconnect an electrical appliance to the apparatus and thus to the AC power source. At least one voltage surge absorption element is interconnected between at least two of the lines for diverting transient voltages, accordingly, this voltage surge absorption element is disposed between the AC power source and the electrical connector(s). Finally, the outlet strip apparatus contains a multi-stage EMI filter interconnected in series with the first and second input line and grounded to the ground line. The multi-stage EMI filter is disposed between the AC power source and the electrical connector such that electricity from the AC power source to the appliance may be filtered and such that electricity returning from the appliance to the AC power system may again be filtered.
As noted above, the apparatus"" input lines are interconnectable to an AC power source. In one embodiment the first input line is connectable to an AC power line while the second input line is connectable to an AC neutral line. In this embodiment, the apparatus may be configured such that it uses a 120 VAC current. Alternatively, both the first and second input lines may be connectable to an AC power line such that the apparatus is able to carry 220 VAC, as is standard in many European countries, as well as a standard for some electronic goods, such as direct current motor treadmills, dryers etc. As will be appreciated, in either of these embodiments, the apparatus will be adapted to interconnect with an appropriate outlet (i.e., 120 VAC or 220 VAC). Accordingly, depending on what configuration is used, the apparatus"" electrical connector will be configured such that it interconnects electrical appliances which use the correct voltage.
The apparatus"" voltage surge absorption element typically will connect at least two of the apparatus"" lines (i.e., the first input line, the second input line and the ground line). As will be appreciated, one end of the voltage surge absorption element will generally be attached to the AC power line such that if a large transient voltage (i.e., spike) enters the apparatus, a predetermined resistance within the voltage surge absorption element will be exceeded, allowing the voltage spike to be diverted to another line (e.g., ground or neutral), thus by-passing and protecting the attached electrical appliance. The voltage surge absorption element may comprise any element that provides sufficient diversion of transient voltages such as a thyrector, a spark-gap or a varistor. In a preferred embodiment the voltage surge absorption element comprises a metal oxide varistor. In addition, multiple varistors may be used to connect all of the lines. For example, a 120 VAC system may use three varistors where the first varistor is connected between the AC power line and the neutral line (l-n), another varistor is connected between the AC power line and ground (l-g) and a third varistor is used to connect the neutral line to the ground line (n-g). These varistors may be chosen depending on a proposed application for the apparatus. For example, 220 VAC systems may utilize a 250 volt 10 amp (V250LA10), whereas for 120 VAC systems a 130 volt 20 amp (V130LA20) varistor may be used.
The apparatus"" multi-stage EMI filter will generally be disposed between the voltage surge absorption element and the electrical connector. A multiple stage EMI filter is used to provide added EMI attenuation of conducted EMI originating from the attached appliance. By passing the conducted EMI through multiple filter stages it may be substantially reduced or eliminated prior to reentering the AC power system. As will be appreciated, this prevents this EMI from being conducted into a digital device attached elsewhere in the AC power system and also prevents the AC power system form transmitting this EMI as RFI throughout a structure.
In a preferred embodiment, the multi-stage EMI filter will be a passive EMI filter. Passive EMI filters typically comprise a plurality of inductors connected in series with the input lines (i.e., AC power line and neutral line in a 120 system and the two AC power lines in a 220 VAC system) and may further contain a plurality of capacitors connected between the two input lines and/or to ground. In one embodiment, this passive multi-stage EMI filter will comprise an EMI filter which is isolated in a metal case. In this embodiment, the metal case may be grounded to the system""s ground line. This xe2x80x98sealedxe2x80x99 EMI filter arrangement further reduces noise in the system, since any electromagnetic radiation entering the metal case will be unable to radiate and will eventually be absorbed to ground.
Generally, the multi-stage EMI filter will be chosen from a set of filters that are operative to suppress and substantially reduce common mode (i.e., line-to-ground) and differential mode (i.e., line-to-line) EMI in a predetermined frequency range to increase the total amount of EMI suppressed. When the apparatus is designed for use in suppressing EMI such that digital communications are not interrupted, the filter is operative to suppress EMI in a range between 25 kHz and 20,000 kHz and to attenuate common mode and differential mode EMI in a range between 3 decibels and 65 decibels. Generally, the filter will suppress or attenuate higher amounts of EMI at higher frequencies. For example, an EMI filter may only attenuate EMI by 3 decibels at 100 kHz, whereas it may attenuate between 55 and 65 decibels for all frequencies between 500 kHz. and 20,000 kHz. In this regard, the filter may be operative to effectively eliminate EMI caused by motor driven appliances in a frequency range where consumer digital devices are highly susceptible. However, it will be appreciated that it is possible to utilize EMI filters which attenuate EMI in other frequency ranges in different settings.
In accordance with another aspect of the present invention, a method is provided to filter EMI from AC power to and from an electrical appliance while providing transient voltage surge suppression for that appliance. The method is performed by first interconnecting an EMI filter and a transient voltage surge suppression element (TVSS) between an appliance and an AC power source. Once the EMI filter and TVSS are interconnected, transient voltages from the AC power source having an amplitude greater than a predetermined maximum value are diverted such that these transient voltages by-pass the electrical appliance. The method also includes the step of passing the xe2x80x98incomingxe2x80x99 electricity from the AC power source through an EMI filter such that EMI contained therein may be attenuated prior to reaching the electrical appliance. Finally, the electricity returning from the electrical appliance is also passed through the EMI filter to attenuate any EMI that may have been generated by the electrical appliance itself. For example, where the attached appliance is a motor driven appliance (e.g., a fan) initially switching the fan on may create an impulse spike (i.e., conducted EMI) in the return current. As will be appreciated, if not filtered, this impulse spike may be conducted or radiated to other electrical devices upon reentering the AC power system (e.g., a home""s wiring system). Passing this return current through the EMI filter may attenuate those impulse spikes to tolerable levels.
The passing steps (i.e., to and from the appliance) may further comprise passing the electricity through a multistage EMI filter to provide improved filtering of the electricity. Preferably, when the present method is used for reducing or eliminating EMI that may interfere with a telephony digital subscriber line, the electricity will be filtered to substantially reduce or eliminate EMI occurring in the frequency range between 25 kHz and 20,000 kHz.